Hearthstone is facing down difficult problems and has no perfect solutions in the offing. Much like Magic did in years gone past Hearthstone has the difficulty that the game inevitably becomes more and more difficult to get into for new players and that will eventually erode the playerbase. You can't get your friends to join up and have fun if they need five thousand bucks worth of cards to even really play the game. Hearthstone isn't that bad yet, but it will be with time unless something big changes.
One of the people designing Hearthstone, Ben Brode, posted a video talking about this issue. I do like that he talked about how the team is really aware that releasing more paid content over time will eventually destroy their ability to recruit new players but sadly he wasn't willing to admit that the only solution is known, and will make a lot of people angry. There was a lot of wishful thinking in the video and a serious lack of answers, and that makes sense because he is really stuck between two equally unappealing options:
1. Screw the newbies. Keep releasing content over time. Let new people pay for it, or not, but don't worry too much about it. This means that being competitive has a ever increasing cost and that is going to eventually dry up the supply of new blood.
2. Screw the veterans. There are two ways to approach this but they end up in the same place. Either you have big discounts on old sets and keep the investment required to get into the game the same or you rotate out old content so that older cards simply aren't usable anymore, or aren't usable in most standard formats. In either case people who paid in at the beginning will be really angry that their investment is worthless or you discourage people from buying anything because they know the price will drop. Both are bad for business.
So what is hearthstone going to do?
I suspect they will do something like what Magic does and try to have a hybrid. That is, they will let people play any cards they want in games against the AI, in Brawls, and in random games against other players. However, they will have a separate Anything Goes! bracket and a New Sets Only bracket for competitive pvp play, which will be analogous to Classic and Standard for Magic players. The new players won't be able to be successful in Anything Goes! without a massive investment but they can be competitive in New Sets Only.
This strategy also means that anyone who wants to continue to be able to play at any kind of high level has to buy every single set as it comes out. You can't skip them because the New Sets Only venue will only have a couple sets in it and you will need them all to be competitive. Ideally you want players to pay to play, but you also want some people who contribute minimally to the bottom line if they can recruit other people. Getting people to play games is very much about the number of their friends that already play, so you really want to have everyone play even if they refuse to invest real cash.
I suspect the people running Hearthstone are going to hold fast to the 'we will come up with some kind of magic solution' line as long as is humanly possible. They don't want to admit that their current structure is unmaintainable because that might suppress card sales in the short term.
Eventually though they are going to have to admit that there isn't a perfect solution but there is a passable solution and Magic already figured it out.